diff --git a/BUILD.gn b/BUILD.gn
index 806367e..32d7886 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -42,7 +42,10 @@
   }
 
   if (is_win) {
-    cflags += [ "/wd4267" ]
+    cflags += [
+      "/wd4267",
+      "/wd4701",
+    ]
   }
 
   if (is_clang || is_posix) {
diff --git a/third_party/BUILD.gn b/third_party/BUILD.gn
index ec6f8cb..f899bd1 100644
--- a/third_party/BUILD.gn
+++ b/third_party/BUILD.gn
@@ -252,11 +252,24 @@
   ]
 }
 
+config("fx_libopenjpeg_warnings") {
+  visibility = [ ":*" ]
+  if (is_win) {
+    cflags = [
+      # Signed/unsigned comparisons.
+      "/wd4018",
+    ]
+  }
+}
+
 source_set("fx_libopenjpeg") {
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [
     "//build/config/compiler:no_chromium_code",
     "//third_party/pdfium:pdfium_config",
+
+    # Must be after no_chromium_code for warning flags to be ordered correctly.
+    ":fx_libopenjpeg_warnings",
   ]
   sources = [
     "libopenjpeg20/bio.c",